Hey Randy,
The biggest advantage (in my book.
) to the "nail-head" vs a "tulip" is the additional port area presented in the bowl on the backside; though this value observation is of typical American "inline-valve" cylinder heads often presenting a rather low port presentation and short-turn to the back-side of the valve; whereas for example hemi-heads demonstrating steeper port presentations with more direct shot to the back of the valve do better with the greater back-side angles of the "tulip" valves.
In my experience (again!
) of porting and flow benching work, a superior flow value of a 30° or a 45° seat angle has more to do with the leading and trailing angles and distances on each side of the seat angle involved (but there are other influences such as port bowl choke below the valve seat machining and chamber roof profile) not truly that one versus the other is just always better, and unfortunately this often may have more to do with the condition of the current seats and with these old castings also previous work done by others post the O.E.M.. Remember, at this point in time, often much of this work is as much of a "salvage" operation as it is one of performance betterment and the sum of material to work with is somewhat limited, this meaning more, particularly on rare and/or valuable castings, when referring to removal of seat casting material, isn't always a better idea. Also be sure that the tulip valve, particularly when head diameter is cut-down from something greater, that there truly is a back-side profile that will accept the 30° seat angle and permit an undercut angle to define the seat width and position in the relationship to the cylinder head.
Then there is the fact that the velocity of the incoming mixture volume, as it enters the cylinder, is at it's highest as crossing the valve and cylinder head seating surfaces (point of constriction), so this translates into a certain sum of directional control for this stream into the cylinder thru seat angles. Awful lot of considerations here, including for example valve shrouding by the chamber and cylinder walls; but for thought, I was told by a quite knowledgeable guy ages ago that generally 30° seats were deemed superior at lower engine speeds hence mixture velocities for better dispersion of the mixture thru-out the cylinder, and the 45° was deemed superior at greater engine speeds with faster piston motion down the cylinder coupled with the higher port velocities had, and that even if less efficient in the initial distribution within the cylinder the higher piston velocities with greater turbulence on the up-stroke aids in offsetting such.
But in answer to your question, in my experience (oh-boy!
), it seems most have chosen to use "Nail-Head" valves cut to 45° for the small valve FE's; they're lighter in weight too! But I wouldn't like the idea of changing the 30° seats to 45° in a set of "virgin" heads, and I do agree with the idea of the exhaust valve having more valve head back-angle dimension.
Scott.